Method for the spraying of electron emitting thermionic cathodes



y 1960- w. GRATTIDGE ETAL 2,943,957

METHOD FOR THE SPRAYING 01-" ELECTRON EMITTING THERMIONIC CATHODES FiledOct. 21, 1958 Inventors:

Wa/fer Graft/dye Fredr/lr Ar/hur Muller,

The/r Aflorne v,

METHOD FOR THE SPRAYWG 6F ELECTRON EMITTING THERMIONTC CATHODES WalterGrattidge and Fredrik Arthur Muller, Schenectady, N.Y., assignors toGeneral Electric Company, a corporation of New York Filed Oct. 21, 1958,Ser. No. 768,654

11 Claims. (Cl. 117-223) 2,943,957 Patented July 5, 1960 which exhibitsthe improved characteristics of strong ad'- herence to the underlyingbase, mechanical strength, high density, small particle size and also, asmooth surface.

Other and further objects and advantages will appear from a perusal ofthe following detailed description considered with the accompanyingdrawings in which:

Fig. 1 illustrates one arrangement of apparatus which may be utilized inone embodiment of our improved method,

Fig. 2 illustrates one form of nozzle construction which can be utilizedto spray the coating according to our improved method, and

Fig. 3 illustrates a second embodiment of our improved method.

Referring to the embodiment of our invention illustrated in Fig. 1, agaseous compound such as compressed Many present day electron tubesdepend for their a source of electrons upon the heating of a coatedmetal cathode made of nickel with specific additives such as aluminum,magnesium, titanium, and silicon. The coatings applied to this cathodemetal usually consist of compounds of the alkaline earth elements suchas barium,

strontium, and calcium, or mixtures of such compounds in appropriateproportions. The usual method of preparation of such coatings involvesthe spraying onto the cathode of a single carbonate or mixtures ofprecipitated carbonates from a solution of these materials in somesuitable binder. This spraying operation is performed prior to assemblyof the various tube electrodes and the carbonate coatings are convertedto oxides by heating the electrode during the evacuation process for thetube.

This previous method and others similar to it have certain inherentdisadvantages which result primarily from the formation of largeirregularly shaped crystals of the carbonate. These disadvantagesinclude lack of adherence of the carbonate to the surface, high relativeporosity, high surface roughness, and poor hardness and mechanicalstrength qualities. result in the flaking of the coating or the liftingof the coating away from the metal base before or during conversion ofthe carbonate coating to the oxide. The high The lack of adherence maysurface roughness is detrimental to good operation of improved method ofspraying a cathode coating on the cathode supporting surface.

In accordance with one form of our invention, these enumeratedditficulties and disadvantages are avoided by spraying very finelydivided droplets of a chemical compound towards the surface to be coatedand simultaneously propelling a second gaseous or vaporized compound toconverse with the first in the region preceding the work surface tofacilitate contact and interaction between these compounds to take placeprior to contact with the work surface but after formation of the spray.This reaction produces the final compound which is to be deposited onthe surface to be coated and during transit the vehicle carrying thereaction product is partially evaporated. Such a method produces a largenumber of small particles of coating material to produce a coating air,from a source free of carbon dioxide, is supplied to the spray gun 3through the communicating conduit 2.

By aspirator action, the compressed air acts as a vehicle and thesolution in container 4 is drawn through the communicating passage 5into the spray gun Where it is atomized or converted into smalldroplets, preferably of approximately 1 mil in diameter, by the nozzleapparatus shown generally at 6. The droplets are propelled towards theobject 10 which is preferably a cathode base to be coated. The apparatusdesignated by the numerals 7 and 8 represent nozzles for supplying asecond gaseous or atomized compound which is directedto converge withthe mist or spray from the spray gun approximately at the region 12which is somewhat removed from the surface to be coated. The compoundfrom nozzle 6 and the second compound from the nozzles 7 and 8 react inregion 12 so as to produce the third compound which is to be depositedon the surface 10. A pair of heat lamps are directed at the cathode base10 to heat the surface of the cathode exposed to the spray and beingcoated and to heat the droplets in the latter portion of their distanceof travel, supply the necessary energy to evaporate the solvent for thefirst solution which is used to form the vapor droplets.

In accordance with a specific form of our invention, air which is madesubstantially inert by the removal of carbon dioxide therefrom, issupplied to the spray gun 3 through the communicating conduit 2 to beused as the propelling medium. By aspirator action, a saturated watersolution of barium hydroxide, which is contained at 4, is drawn throughthe communicating conduit 5 into the spray gun where it is convertedinto a fine spray that is carried towards the surface 10 by thepropelling air medium. Two streams of carbon dioxide are directedtowards the intersection region 12 from the nozzle apparatus shown at'1' and 8. The carbon dioxide reacts with the barium hydroxide containedin the droplets so as to form very small insoluble crystals of compoundsof barium, such as barium carbonate or barium bicarbonate. Thesecrystals are carried directly to the surface of thermionic cathodeelement it) which may be from two to three feet away from the region 12,and are deposited thereon. The minute size of the droplets facilitates aquick and complete reaction between the barium hydroxide solutionthereof and the carbon dioxide gas in a fraction of the distance betweenregion 12 and surface 19 whereby the Water in the droplets is partiallyevaporated and the barium carbonate precipitate is added to the cathodesurface in minute increments. The radiations from the heat sources iserve to evaporate the remainder of the Water solvent for the bariumhydroxide which dries the surface and also insures surface uniformity bypreventing running of the deposited coating. The minute amount ofprecipitate in the different droplets enables a fine and even deposit toproduce a smooth and even coating.

performing the method of our invention, various compounds may beutilized in the manner described. Solutions of compounds of alkalineearth elements or any combinations of these may be utilized in nozzle 6while carbon-dioxide, solutions of ammonium carbonate or anyof'the'substituted ammonium carbonates such as monoet hanolamine or anycombinations of these solutions are utilizable in the nozzles 7 or 8;The reaction product is' a carbonate precipitate depositable on thecathode surface; Although various alkaline earth compounds are conceivedas useful in performing our method, compounds considered particularlyuseful are acetates, chlorides, formates, hydrates and nitrates ofbarium and strontium. As shown in Fig. 2, a nozzle, designated generallyat -13, is effective in accomplishing the improved spraying method ofour invention and may includepro'pellant carrying conduit member -14coaxially surrounding one end of communicating conduit 2 and extendingbeyond the end of conduit 2 by 'a distance designated A in the drawings.The open 'end'of conduit 13 converges to opening 15 of dimensiondesignated B and conduit 2 converges to an opening 16 of dimensiondesignated C. 'It has been experimentally determined that the opening16, if his the range of 4 to 10 mils in diameter, will supply dropletsof the alkaline earth hydroxide approximately l rn'il 'indi'amet'erwhich has been found to be a satisfactory particle size in accordancewith our improved method. The dimensions A and B become particularlyimportant if the second reagent is to be used as the propelling mediumas is set forth in the second embodiment of this invention. The distanceA is selected so as to limit the reaction time within the spray gun andthe dimension B is selected so as to be sufliciently large as not toclog with particles formed during the reaction time determined by thedistance A. According to one embodiment of our invention the dimension 3has been determined experimentally to be within the range 10 to 40 milsand the dimension A has been found to vary the range of 20 to 40 mils. Anozzle, which has been successfully used with either of the illustratedembodiments of this improved method, has the dimensions of '4 mils foropening 1 6, 20 mils for opening 15, and 25 mils for distance A. Using anozzle with opening B of substantially mils, a coating of .6 mil at adensity of 1.8 gr./cc. may be applied in 10 to minutes of spraying time.It implicit in our invention that the smallest droplet possible beutilized in accordance with the reasonable spray speed which may bedesired; that is, the larger the droplet is, the less chance there is offull reaction with the secondreagent before contact with the surface. Itis seen, therefore, that the distance from the gun to the work surfaceis also determined by the degree of complet'eness desired in thechemical reaction. Thus, a wide range of dimensions of nozzle componentsand spacings between the spray nozzle and the surface to be coated canbe utilized without departing from the spirit of this invention. It isan essential feature of this invention, however, that no carbon-dioxidebe drawn into the storage vessel for the barium hydroxide. That is,there should be no reaction between the two reagents in any storagevessel.

Referring to Fig. 3, a modified apparatus is shown wherein thepropelling gas for the alkaline earth hydroxide is also one of thereacting compounds in the improved method. This propelling gas issupplied from the container 21 through the communicating conduit 22 tothe aspirator spray gun 23 which draws a portion of the second liquifiedcompound in the container 24, through the communicating conduit 25 tothe spray gun 23 where a fine spray is created from the two compounds bythe nozzle apparatus shown generally at 26. These two compounds reactduring the course of travel from first contact Within the spray gun tothe surface to be coated 28. The relative nozzle dimensions, aspreviously described with respect to Fig. 2, are maintained in thisembodiment so that the particles formed by the reaction, when depositedon a surface 28, will be of optimum size for proper coating. The heatsource 29 can be used to accelcrate the drying process of a depositedcoating and to further insure the uniformity of the cathode coating bypreventing undesirable effects from running by a fluent deposit.

A specific example embodying the principles disclosed by the apparatusof Fig. 3 would involve the use of carbon dioxide from the container 21as the propelling me dium and being supplied to the spray gun '23through the communicating conduit 22. The aspirator action of the spraygun draws the barium hydroxide, which is conface 28 to be coated. Thebarium hydroxide and carbon dioxide react along the path of travel toproduce barium carbonate which is deposited on the surface 28. Theradiant heat energy source 29 serves to evaporate the solvent from thebarium hydroxide. i

The mechanical strength of the type of coating pro duced by our methodallows for filing or other mechanical abrasion to produce coatings ofany requisite thickness. For example, coatings of substantially .6 milhave been sprayed on nickel cathodes and then filed to a coat: ingthickness of .3 mil. Such a filed coating has been used in a triode withresultant improved noise figures.

Having thus described this invention in such full, clear, concise andexact terms as to enable any person skilled in the art to which itpertains, to make and use the same and having set forth 'the best modecontemplated of carrying out this invention, we state that the subjectmatter which We regard as being our invention is particularly pointedout and distinctly claimed in what is claimed, it being understood thatequivalents or modifications of or substitutions for parts of the abovespecifically de scribed embodiment of this invention may be made withoutdeparting from the scope of the invention as set forth in what isclaimed.

What we claim as new and desire to secure by Letters Patent of theUnited States is: a

1. The method of coating a cathode base with an electron emissivematerial comprising the formation of small droplets of a hydroxidesolution of alkaline earth elements, propelling said droplets towardsthe cathode base to be coated, directing a flow of carbon dioxide towardsaid droplets for producing an inter-mixture therebetween duringtransit, the reaction of the hydroxide contained in said droplets withcarbon dioxide forming a carbonate of the alkaline earth metal prior tocontact with the cathode base to be coated, evaporating the liquid insaid droplets, and deposition of said carbonate on the cathode base. V

2. The method of coating a cathode base with an electron emissivematerial comprising the formation of small droplets of an hydroxidesolution of alkaline earth ole-t ments, propelling said droplets towardsthe cathode base to be coated, enveloping said droplets in transit in acloud of carbon dioxide to effect a reaction forming carbonate of thealkaline earth metal in each of said droplets, said reaction takingplace after formation of the droplets but before contact with thecathode base to be coated and depositing said carbonate on the cathodebase;

3. The method of coating a cathode base with an electron emissivematerial comprising the formation of small droplets of an hydroxidesolution of alkaline earth elements substantially one mil in diameter,propelling said droplets towards the cathode base to be coated,enveloping said droplets in a cloud of carbon dioxide to elfe'ctareaction forming a carbonate of the alkaline earth metal, said reactiontaking place after formation of the droplets but before contact with thecathode base to be coated,

and heating the droplets of formed carbonate prior to and after contactwith said cathode base.

4. The method of coating a cathode base with an electron emissivematerial comprising the formation of small droplets of an hydroxidesolution of alkaline earth elements substantially one mil in diameter,propelling said droplets towards the cathode base to be coated, reactingthe alkaline earth element hydroxide contained in said droplets withcarbon dioxide to form a carbonate of the alkaline earth metal prior tocontact with the surface to be coated, and depositing said carbonate ofthe alkaline earth metal on the cathode base to be coated.

5. The method of coating a cathode base with an electron emissivematerial comprising the steps of forming a spray of droplets of bariumhydroxide solution, propelling said droplets toward a cathode base to becoated, enveloping said droplets in a cloud of carbon dioxide duringtransit toward said cathode base, said droplets and carbon dioxidereacting to form a precipitate of barium carbonate deposited on saidsurface, evaporating a portion of the liquid in said droplets duringtransit and drying said surface after deposition.

6. The method of coating a cathode base with an electron emissivematerial comprising the steps of forming a spray of droplets of asolution of barium and strontium hydroxides, propelling said dropletstoward the cathode base to be coated, enveloping said droplets in acloud of carbon dioxide during transit toward said surface, saiddroplets and carbon dioxide reacting to form a precipitate of barium andstrontium carbonates deposited on said cathode base and drying saidcathode base during transition and after deposition on said cathodebase.

7. The method of coating a cathode base with an electron emissivematerial comprising the steps of forming a spray of droplets of asolution of barium hydroxide, propelling said droplets toward thecathode base to be coated, forming a spray of droplets of a solution ofammonium carbonate and propelling the same to converge with the dropletsof said first solution at a region adjacent to the cathode base, thedroplets of different solutions reacting to form a precipitate of bariumcarbonate and depositing said precipitate on the cathode base to becoated.

8. The method of coating a cathode base with an electron emissivematerial comprising the steps of forming a spray of droplets of asolution of ammonium carbonate, propelling said droplets toward thecathode base to be coated, forming a spray of droplets of a solution ofbarium acetate and propelling the same to converge with the droplets ofammonium carbonate solution at a region adjacent to the cathode base,the droplets of different solutions reacting to form a precipitate ofbarium carbonate and depositing said precipitate on the cathode base tobe coated.

9. The method of coating a cathode base with an electron ernissivematerial comprising the steps of producing a spray of small droplets ofa solution of a compound selected from one of the group consisting ofacetates, formates, hydrates, chlorides and nitrates of alkaline earthelements, propelling said droplets toward the cathode base to be coated,mixing the solution in said droplets during transit with a carbonatecompound at a region removed from said cathode base, and therebyproducing as a precipitate a carbonate derivative of said one compoundof said group and depositing and drying said precipitate on said cathodebase.

10. The method of coating a cathode base with an electron emissivematerial comprising the steps of producing a first spray of smalldroplets of a solution of a compound selected from one of the groupconsisting of acetates, chlorides, formates, hydrates and nitrates ofalkaline earth elements, propelling said droplets toward the cathodebase to be coated, intermixing with said first spray, a second spray ofsmall droplets of a carbonate compound and thereby producing as aprecipitate, a carbonate derivative of said one compound of said groupand depositing and drying said precipitate on said cathode base.

11. The method of coating an electron emissive coating comprising thesteps of producing a spray of small droplets of a combination ofsolutions of a compound selected from the group consisting of acetates,chlorides, formates, hydrates and nitrates of an alkaline earth element,propelling said droplets toward the surface to be coated, mixing thesolutions in said droplets during transit with a carbonate compound at aregion removed from said cathode base, and thereby producing as aprecipitate, a carbonate derivative of said compound of said group anddepositing and drying said precipitate on said cathode base.

Peacock Sept. 10, 1940 Fischer Aug. 8, 1944

1. THE METHOD OF COATING A CATHODE BASE WITH AN ELECTRON EMISSIVEMATERIAL COMPRISING THE FORMATION OF SMALL DROPLETS OF A HYDROXIDESOLUTION OF ALKALINE EARTH ELEMENTS, PROPELLING SAID DROPLETS TOWARDSTHE CATHODE BASE TO BE COATED, DIRECTING A FLOW OF CARBON DIOXIDE TOWARDSAID DROPLETS FOR PRODUCING AN INTERMIXTURE THEREBETWEEN DURING TRANSIT,THE REACTION OF THE HYDROXIDE CONTAINED IN SAID DROPLETS WITH CARBONDIOXIDE FORMING A CARBONATE OF THE ALKALINE EARTH METAL PRIOR TO CONTACTWITH THE CATHODE BASE TO BE COATED, EVAPORATING THE LIQUID IN SAIDDROPLETS, AND DEPOSITION OF SAID CARBONATE ON THE CATHODE BASE.